The present invention is directed to nonwoven fabrics and particularly to medical fabrics. The term "medical fabric", as used herein, is a fabric which may be used as the fabric for surgical drapes, surgical gowns, instrument wraps, or the like. Such medical fabrics have certain required properties to insure that they will perform properly for the intended use. These properties include strength, the capability of resisting water or other liquid penetration, often referred to as strike-through resistance, as well as being breathable, soft, drapable, sterilizable and a bacterial barrier.
In important areas, some of these characteristics or attributes appear to be in direct opposition in one another in a given fabric, and consequently, conventional disposable medical fabrics have not achieved an optimum balance of the desired attributes. For instance, with prior art fabrics which are comprised predominantly of spunbonded webs, random-laid staple fiber webs, tissue and scrim laminates, spunlaced webs, or combinations thereof, attempts to improve the key repellency or barrier properties invariably compromise breathability and softness attributes. Thus, those fabrics with superior breathability and other aesthetic attributes tend to be relatively inferior in strike-through resistance. As a further example, in order to improve strength, it is generally known to increase the weight of the fabric, compact the web to a high degree, add or increase binders, or adopt various combinations of these techniques. However, such attempts often result in undesirable aesthetic characteristics, as the resulting fabrics tend to be generally stiffer and less breathable.
The use of microfiber webs in applications where barrier properties are desired is known in the prior art. Microfibers are fibers having a diameter of from less than 1 micron to about 10 microns. Microfiber webs are often referred to as melt-blown webs as they are usually made by a melt blowing process. It is generally recognized that the use of relatively small diameter fibers in a fabric structure should allow the achievement of high repellency or filtration properties without undue compromise of breathability. Microfiber web fabrics made heretofore, and intended for use as medical fabrics, have been composites of microfiber webs laminated or otherwise bonded to spunbonded thermoplastic fiber webs, or films, or other reinforcing webs which provide the requisite strength to the fabric. The microfiber, melt-blown webs heretofore developed have been reported to possess insufficient strength, when unreinforced, to be used in medical or other applications where high strength at relatively low weights is desirable or important. For example, in the "Journal of Industrial Fabrics", Vol. 3, No. 1, 1984, pages 33-44, it is indicated that the most serious deficiency of melt-blown webs is their strength per unit weight when compared to conventional webs or scrims of the same material. Japanese Patent Application Disclosure 180, 653-1983 indicates that melt-blown sheets lack tensile strength and attributes this deficiency to the fact that the melt-blown sheets are formed of undrawn fibers. United Kingdom Patent Application Disclosure G.B. No. 2104562A, discloses that other fibrous reinforcements are necessary to impart strength to melt-blown webs for use as medical fabrics. U.S. Pat. No. 4,041,203 discloses a nonwoven fabric made by combining microfiber webs and spunbonded webs to produce a fabric useful as a medical fabric. The patent gives various examples demonstrating the necessity of reinforcing melt-blown webs to provide adequate strength. Example 1 of the patent describes a thermally point-bonded melt-blown web before and after addition of a spunbonded reinforcement layer. The grab tensile strength to basis weight ratio of the embossed unreinforced melt-blow web was lower than can be used for medical fabrics at practical weight levels. The patent further specifically discloses various webs made with polypropylene. The microfiber webs and the continuous-filament, polypropylene, spunbonded webs are laminated together through the application of heat and pressure in a pattern to produce a medical fabric. The continuous-filament, spunbonded web provides the strength to the laminated fabric.
U.S. Pat. No. 4,302,495 discloses a laminate of a microfiber web and a directionally-oriented thermoplastic netting. The thermoplastic netting provides the requisite strength to the finished nonwoven fabric.
U.S. Pat. No. 4,196,245 discloses combinations of melt-blown or microfine fibers with apertured films or with apertured films and spunbonded fabrics. Again, the apertured film and the spunbonded fabric are the components in the finished, nonwoven fabric which provide the strength to the fabric.
While the above-mentioned fabrics have the potential to achieve a better balance of repellency and breathability compared to other prior art technologies not using microfibers, the addition of reinforcement layers of relatively large diameter fibers limits their advantages. The fabrics have to be assembled using two or more web forming technologies, resulting in increased process complexity and cost. Furthermore, the bonding of relatively conventional fibrous webs to the microfibers can result in stiff fabrics, especially where high strength is desired.